Snowboarders and marine engineers both worry about avalanches, but the latter may have a tougher job when working underwater. They have to understand not only what makes a cliffside collapse, but also how fluid between sand grains affects the flow.

Physicists have now simulated how that process happens. A team led by Farhang Radjai of the University of Montpellier in France developed calculations for how particles flow when they collapse from a tall stack. Different simulat...